By encountering PoE Trade creatures which can be trapped in ice crystals you can obtain essences. The monsters are released by you by clicking on the crystals several times, the centre one conveys the essences. After you defeat the monster, the essences it is carrying drop. In the image below you can see when you encounter them while 17, how character monsters seem.
In the image you can see Weeping Essence of Doubt -- let me clarify a little:"Weeping" is prefix and it reflects the tier of the character. Other tiers you might notice is wailing, crying, whispering, muttering etc."Of Doubt" is a suffix and it represents type of the character -- different types of essence offer different mods. By way of example, Essence of Woe offers energy defense mods when crafting armour but Essence of Greed -- lifetime mods.
To estimate path of exile wiki:"Corrupted from the entire world of Wraeclast, several exiles have gone rogue, making them turn off their own kind." . Rogue exiles are monsters that use a few of the abilities that players may use -- lightning arrow, totems, flameblast, leap slam to list a couple. They can appear in the playthrough -- even Act 1. Some are more dangerous than others so players may get amazed by how much damage they could deal. When you're coming T11-T16 maps exiles inside these maps can be extremely deadly, so be skeptical of that.
From the picture, it is possible to view duelist Rogue Exile with title Armios Bell who utilizes Strike and Flicker Strike to Buy PoE Currency assault. When slain, Rogue exiles constantly drop an item.